Between 1937–1940, Japanese colonial engineers displaced approximately seventy thousand Chinese and Korean farmers along the Yalu River to make way for a massive new hydroelectric project: the Sup’ung Dam, second largest in the world at the time of its completion. Colonial officials later brought new tenants to the now-submerged spaces farmers formerly occupied. These were millions of fish, whose scaly bodies were intended to fill the caloric needs of a growing Japanese empire in what was called the “world’s number one fish hatchery.” Construction of the Sup’ung dam led to a fundamental transformation of the Yalu’s river ecology and a reassessment of its piscatorial possibilities. The Sup’ung dam negatively impacted the Yalu’s pre-existing freshwater fisheries. Yet it also brought an unprecedented amount of official and scientific attention to the river’s underwater resources. Wartime exigencies drove Japanese colonial regimes in Korea and Manchukuo, previously divided over fishing rights, to cooperate on aquacultural development of the Sup’ung reservoir fishery. Japan’s defeat in the Second World War ultimately interrupted these plans. The fish planted there by colonial authorities, however, continued to feed hungry populations into the post-colonial era and set a precedent for profitable fisheries development that occurred in the reservoir after 1959. Postwar regimes in China and North Korea disavowed the legacies of Japanese colonialism, but they shared a logic of development that valued the aquacultural potential of the Sup’ung reservoir over the river’s native fisheries.